NPT Tapping

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NPT Tapping

NPT Tapping

Hello,I am wondering about tapping a 1/4" NPT threads. The drawing from my customer shows the hole 18mm deep and tapped this depth. I see from the handbook the tapping size is 7/16". when i drill my hole then go to tap it, the tap won't go down this depth, maybe only 10mm. Should the hole be tapered ?Also he has given me a dwg. of a 10mm thick plate with a hole drilled & tapped 1/8" NPT. I assume/guess i just tap from one side as far as the tap will go?? There must be some standard that these thread depths should be. any help appreciated.

RE: NPT Tapping

Short projection pipe taps are made for shorter length pipe threads, only a few companies make them. They will require a different tap drill and may have to be reamed with a special reamer before tapping harder material.J&L Industrial, www.jlindustrial.com, and www.mscdirect.com have them in their catalogs.

RE: NPT Tapping

By the way, note that length of effective thread is not the same as required tap depth. My data shows that your blueprint callout of 18mm tap depth is within the normal tolerance range for 1/4 NPT (standard tolerance is +- one thread).

RE: NPT Tapping

Thanks guys,I think mrainey's last point is a key issue. "length of effecive thread is not the same as required tap depth." looking at your link http://bellsouthpwp.net/r/s/rsnmar/METhreads.zipas you have allready pointed outit seems that the tap depth of 18mm shown on my drawing is correct. the chart says 17.445mm.So basically i'm back to my original problem.so with your help, we've estblished:1)Tapping Drill 7/16"2)Tapping Depth 17.5mm approx.3)effective thread length of 0.402" or 10.2mmthe only way i can see of achieving the tapping depth is to increase the tapping size! or some way make a tapered hole! when the tap gets down approx.10mm then the top of the hole is smaller than the root of the tap.Thanks guys.

RE: NPT Tapping

To make certain NPT threads are the correct depth, a thread plug gage is needed. Some gages are marked only for "basic" depth, and some have min and max limits also. Without a gage, I would tap the holes until 6 full threads are remaining on the top of the tap. This will produce an in tolerance hole for almost all taps.

RE: NPT Tapping

You say the hole depth is called out as 18mm. Are you allowing for the drill point? If not, the tap may be bottoming out on the drill point. You might need to drill deep enough to get the full diameter x 18mm deep (if allowed), or drill 18mm deep with the drill then use an end mill or flat-bottom drill to remove the drill point.

RE: NPT Tapping

Markb2,You might ask the customer what his mating part lookslike. Normally the depth of a tapered pipe tap is notshown. His requirement does not make any sense unless heis using some non-standard male thread. The answer mightbe to provide a counterbore at the surface if he iscertain about the depth. Ask if that is an option.

RE: NPT Tapping

I have been thru this many times. What your customer says will determine what you do. Here are two things to know. Use a tapered pipe reamer after the tap drill. This reduces the chip load. Nearly Every tool catalog will have them. The normal or standard pipe thread gauges are not "plus or minus" the ground reference line. Read the explanation that comes with the gauge, Do-all has the one I use.

RE: NPT Tapping

FROM MY EXPERIENCE, THIS IS KNOWN AS A "SHORT PROJECTION" TAP. YOU WILL NEED TO GRIND YOUR TAP TO THE CORRECT LENGTHIE. REMOVE THE LEAD-IN THREADS, TO ACHIEVE YOUR 18MM(.708")NO, THE HOLE IS NOT TAPERED. THE TAP IS TAPERED. THATS'HOW YOU ACHIEVE A SEAL BETWEEN YOUR FEMALE/MALE THREADS.WHERE DOES THE CUSTOMER WANT TO SEAL, AT TOP OR BOTTOM OF HOLE? STANDARDS SAY 1/4-18 NPT IS 7/16"DIA. HOLE X 7/16" DEEP. ROUGH CALCULATIONS YOU NEED TO GRIND OFF .074" OFFTHE END OF YOUR TAP TO ACHIEVE GAGE AT 18MM.

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